Netfilter Modbus/TCP

Modbus/TCP is an open automation protocol used in
a variety of SCADA, PLC, and industrial IO
applications. Modbusfw is a Linux 2.4.x Netfilter
Extension that permits filtering decisions (DROP,
REJECT, etc.) based on application-layer values,
allowing finer-grained access control than is
currently possibly by simply blocking TCP port
502.